by There are more than 500 species of sharks in the world’s oceans, from the 7-inch dwarf lantern shark to whale sharks that can grow to over 35 feet in length. They are found from polar to equatorial waters, on the water surface and miles deep, in the open ocean, along coasts and even in some coastal rivers.
With such diversity, it’s no surprise that sharks serve many ecological functions. For example, the largest individuals of some large prey species, such as tigers and white sharks, may play an overwhelming role in maintaining balance among species. They do this by feeding on prey and sometimes by just being present and frightening enough that prey species change their habits and locations.
In a newly published study, my colleagues and I surveyed decades of research on the ecological roles of sharks and considered their future in human-dominated oceans. We found that because sharks play such diverse and sometimes important roles in maintaining healthy oceans, their current decline is an urgent problem. Since 1970, the global population of sharks and rays has declined by more than 70%.
Humans are killing many types of sharks at unsustainable rates, mainly through overfishing. We see a need for nations to rethink where and how to conserve sharks for healthy oceans.
How sharks feed on sea grass
Along the remote coast of Western Australia, more than two decades of work shows that the mere presence of tiger sharks shapes the entire seaweed ecosystem by changing where and how large grazers, such as sea turtles and sea cow, feeding.
Tiger sharks protect a wide range of grasslands from being overgrazed, allowing it to grow in thick underwater meadows that provide habitat for young fish and shellfish. These species are important food for other animals and humans.
In areas where tiger sharks have declined and turtle numbers have increased, sea otters are overgrazed. In Bermuda, for example, sea turtles have almost completely collapsed due to the exploding turtle population.
White sharks produce some of the same effects. Along the California coast, where white shark populations are increasing, otters are spending more time in the safety of protected inland waters and less time in the open waters of Monterey Bay. The otters prey on crabs, which feed on grazing invertebrates such as sea slugs that scavenge algae from the sea. More otters means fewer crabs, more grass and healthier sea grass.
Kelp forests and reefs
Kelp forests are dense stands of large brown algae that grow in shallow zones near coasts. On the US West Coast, overhunting drove the local sea otter populations to extinction by the early 1900s. This led to massive kelp forest losses when sea urchins – the favorite food of water dogs – were allowed to spread and eat kelp.
Over the past 50 years, otter populations have been decimated by federal protection. But as white sharks expand their ranges north, they are preventing otters from expanding their range because there are no kelp forests for the otters to hide in.
The otters will probably not expand their ranges until kelp forests are established. This complicates restoration efforts, as the otter will not remove enough pole to establish kelp.
When sharks are present near coral reefs, fish avoid the sharks by sticking close to the safety of the reef. This reduces grazing on seaweed and algae over wide areas. However, much remains to be learned about when, where and how sharks may be important to coral reef health.
Food and nutrient sources
Sharks can also be prey. Some, including large species such as white sharks, are important food sources for several killer whale populations around the world. Smaller sharks, including blacktip sharks, can be main menu items for larger sharks, such as great hammerheads.
As sharks eat prey in one place and excrete waste in another, they move nutrients throughout the ocean. In the Pacific Ocean, for example, green reef sharks move nitrogen from the offshore waters where they feed to the coral reefs where they spend their days, providing important fertilizer for ocean food webs.
In Florida’s coastal waters, young bull sharks feed during brief visits to the ocean, then return to safer, near-freshwater rivers, where they spend most of their time and release nutrients in their waste.
Sometimes the presence of sharks helps other fish. In the open ocean, rough shark scales make perfect scratching posts for fish to remove parasites.
Protecting the roles of sharks
Our review clearly shows that sharks play a variety of roles in maintaining healthy oceans. We see important implications for shark conservation.
Step 1 would involve setting targets beyond ensuring that there are sharks in the oceans and focusing on species with key ecological roles.
Within populations, it is important to protect certain types of individual sharks. For example, the largest tiger sharks are the ones that shape the behavior of turtles and manatees, which benefits grassland ecosystems. Due to intensive fishing around the world it is extremely challenging for large sharks that can live for years or even centuries to survive and grow to ecologically important sizes.
By working with local communities in coastal areas, support could be built to protect these large ocean predators, much like conservationists working on land to protect iconic predators such as wolves. Nations could build networks of large protected areas that prohibit shark fishing, targeting key areas where individual sharks might roam.
Research shows that sharks benefit from creating protected areas, limiting the catch of sharks outside these zones and restricting the use of fishing gear that harms sharks the most, such as gill nets and long lines. With a clearer understanding of the ecological value of sharks, my colleagues and I hope to see focused action at all levels to protect these essential animals.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. It was written by: Michael Heithaus, Florida International University
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Michael Heithaus receives funding from the Shark Conservation Fund and the National Science Foundation.
